mtd: st_spi_fsm: fixup Kconfig dependency

I hear that this driver should depend on ARCH_STI, and that "SH" is not
actually a real symbol. At the same time, let's allow compile-testing on
other ARCH'es.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Marek Vasut <marex@denx.de>